Optical disks such as compact disks (CDs), video compact disks (VCDs) and digital versatile disk (DVDs) are playable by optical storage devices. When an optical pickup head of an optical storage device operates, the light emitted by a light source such as a laser diode is focused by an object lens of the optical pickup head on an optical disk, and the light reflected by the optical disk is transmitted to a light sensor to realize information from the disk. Referring to FIG. 1, the optical pickup head 10 moves along two main directions, i.e. a direction perpendicular to the disk surface, referred as a focusing direction F, and a direction parallel to the disk surface, referred as a tracking direction T.
During operation, a focusing error and a tracking error usually arise on the rotating disk. The focusing error is generally caused by vibration of the disk in the focusing direction F, and the tracking error is caused by eccentricity of the disk in the tracking direction T. To correct these errors, a tracking control system for an optical pickup head was developed, as can be seen in FIG. 2. The optical pickup head 20 has six light receiving parts A, B, C, D, E and F for producing output signals, when receiving light reflected from the optical disk. The output signals e and f are amplified through an amplifier 21 to produce a tracking error signal TE.
FIGS. 3(a), 3(b) and 3(c) respectively show three possible situations of tracking error signals. Referring first to FIG. 3(a), an ideal tracking error signal TE1 is an alternating current (AC) signal distributing in a preset amplitude range without any direct current (DC) component effect. That is, the DC level of the AC signal is preferably zero. The waveform of the tracking error signal TE1 is symmetrical with respect to the zero level 0. It is known to those skilled in the art that the zero level, which indicates the best tracking condition, is an index for performing a feedback control. In practice, however, the voltage level of the tracking error signal might be shifted upward or downward due to the presence of the DC effect, as the signal TE2 shown in FIG. 3(b). The tracking error signal TE2, although distributing within the input range of the following A/D converter, has an average voltage Vf, which is deviated from the zero level to some extent. In order to overcome the above-described problem, a tracking balance operation is performed with reference to FIG. 2. After the tracking error signal TE2 is converted into a digital signal by an analog-to-digital (A/D) converter 22, a peak and a bottom values of such digital signal are then detected by a peak detector 23 and a bottom detector 24, respectively. Based on the peak and bottom values, the tracking balance device 25 computes the drift voltage Vf of the FIG. 3(b) to realize a deviation amount of the DC level of the tracking error signal TE2 from the zero level. The deviation amount is then fed back to and offset by the amplifier 21 to remove the DC component effect from the tracking error signal TE2.
In the third situation shown in FIG. 3(c), the tracking error signal TE3 is beyond an upper limit Dmax and a lower limit Dmin of the A/D converter 22. Under this circumstance, correct peak and bottom values cannot be obtained to proceed subsequent DC-level determination and voltage-drift offset operations.